From U.S. Pat. No. 5,848,996 a cannula designed for administering spinal anesthesia is known. Said design features a rigid cannula tube, the end of which exhibits a region convexly tapering to an insertion point as well as two outlet apertures forward to where the tapering region begins. This cannula is described a “pencil point spinal cannula” owing to its pencil-point-shaped tip. The disadvantage of this design, however, is its proportionally large distance between the outlet aperture and the insertion point. When spinal anesthesia is administered, this feature requires the cannula to be inserted as deep as possible in the subarachnoid space until the outlet aperture enters the subarachnoid space. Because of the deep insertion required, injuries to the nerve fibers can occur. Because the outlet apertures are round, their cross section can only be small. As a result, fluid exits the outlet aperture at a proportionally high exit velocity. Furthermore, the manufacture of this known cannula is complicated and expensive.
From DE 199 11 970 AI a cannula of the prior art is known. The insertion point is essentially conical and closed. The at least one fluid passage aperture is formed by a longitudinal slit along a side wall of the shaft. The manufacture of the conical point and narrow slit is complicated. Furthermore, the slit acts as an injector, thereby giving exiting fluid a proportionally high velocity, which is not desired. A method for producing this known cannula is not disclosed. Apparently, the conical point is first produced with the narrow slit being subsequently formed in a special stage.
From U.S. Pat. No. 3,906,932 a cannula point for penetrating a container stopper is known, which when introduced into the stopper is not deflected to one side. To achieve this type of function, two diagonal grindings are made on opposite sides of the end of a cylindrical tube. The thereby resulting points are bent toward one another to form a sheath that facilitates easier penetration of a stopper. A cannula of this type is not suited for medical use, since it would not expand pierced tissue, yet would instead penetrate straight through tissue. The process described in this document is therefore not suited for the manufacture of a cannula for medical use.